In general terms, specific language impairment (SLI) is easy to define: It is diagnosed when a child fails to make normal progress in language learning
for no obvious reason. In practice, however, this simple characterization is
deceptive. Deciding who should or should not be regarded as having SLI
can be fiendishly difficult. In this chapter, I discuss three issues that arise
when defining diagnostic criteria for SLI: (a) the question of whether there
should be a substantial discrepancy between IQ and language level, (b)
comorbidity with and differentiation from other developmental disorders, and (c) heterogeneity of developmental language impairment. Experts differ in their recommendations as to how these issues should be addressed. My own view is that much of the controversy arises because
people are looking for a single diagnostic solution to a range of different
problems.

IS A SINGLE DEFINITION FEASIBLE?

We first need to consider the purpose to which we put our diagnostic definition. When we do so, we find that the optimal way to define SLI varies
according to the context. Many of the early attempts to formulate diagnostic criteria for SLI were made by researchers whose goal was either to discover the underlying cognitive basis of SLI or to characterize the linguistic
deficits of the disorder. In this kind of study, it is important to adopt strin-

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